Protective immunity to infection from encapsulated bacteria is mediated predominantly by antibody (Ab) to their capsular polysaccharide (CP). The human Ab repertoire to the CP of such encapsulated bacteria is often highly restricted in diversity with use of a limited number of immunoglobulin heavy chain variable region (V/H) and light chain variable region (V/L) genes. The ontogeny of the human Ab response to the Haemophilus influenzae type b (Hib) Hib and multiple pneumococcal CP are delayed in the infant, which is the age of highest risk of infection with encapsulated bacteria. The mechanism of this delay in ontogeny is unknown. We hypothesize that contributing to this delay are poor expression of particular V genes and of N region addition, a process of nontemplated nucleotide addition mediated by the enzyme terminal deoxynucleotidyl transferase. N region addition is delayed in ontogeny in the mouse and particularly in murine CD5 positive B cells, which have been implicated in murine Ab production to CP. The aims of this proposal are to: 1) determine whether the neonate's kappa light chain variable region (Vkappa) repertoire is skewed compared to the adult's; 2) determine the ontogeny of N region addition at the Vkappa-Jkappa junction; 3) develop approaches to defining human Ab repertoires to CP. The results should provide insights into ontogeny of human Ab responses and a background for developing vaccines to prevent bacterial infections in infants.